(a) Field of the Invention
The present invention relates to a mobile communication system, and more particularly, to a method of detecting a space-time code.
(b) Description of the Related Art
In general, WiBro, which is a field of a mobile communication system, means a portable Internet system that has two meanings of “wireless” and “wideband”. A Wibro system has been defined as a system that satisfies the standard established by IEEE 802.16 and five performance criteria, by the Ministry of Information and Communication on Jul. 29, 2004. Here, the five performance criteria include that {circumflex over (1)} when a mobile terminal moves at a speed of 60 km/h, a data transmission rate of at least 512 kbps during a downlink and a data transmission rate of at least 128 kbps during an uplink are ensured at cell interfaces, {circumflex over (2)} a channel bandwidth is 9 MHz or more, {circumflex over (3)} a roaming process of communication equipment needs to be performed between service providers, {circumflex over (4)} a frequency reuse coefficient is 1, and {circumflex over (5)} a TDD method is used to minimize interference between frequencies, while transmitting/receiving time slots synchronize with each other.
In the IEEE 802.16e, when there are four transmitting antennas in a system, three usable space-time codes are defined. The three space-time codes are represented by three matrices A, B, and C, respectively, and each space-time code has a different symbol transmission rate and diversity gain. Among them, particularly, the space-time code B has a superior trade-off performance for the symbol transmission rate and the diversity gain, as follows.
  B  =      [                                                      s              1                        -                                          s                2                *                            ⁢                              s                5                                      -                          s              7              *                                                                                                      s                2                            ⁢                              s                1                *                            ⁢                              s                6                                      -                          s              8              *                                                                                      s              3                        -                                          s                4                *                            ⁢                              s                7                            ⁢                              s                5                *                                                                                                    s              4                        ⁢                          s              3              *                        ⁢                          s              8                        ⁢                          s              6              *                                            ]  
In this case, the vertical axis indicates an antenna. It can be understood that four symbols are simultaneously transmitted from four antennas. The horizontal axis indicates time or a carrier frequency.
When symbols are transmitted by using the given space-time code, the symbols are simultaneously received by a receiving terminal. Thus, the entire performance of a system depends on an efficient detection method in the receiving terminal. Among detection methods that have been proposed in the related art, a maximum likelihood (ML) detection method is most superior in terms of performance.
However, the ML detection method is very complicated. In particular, in the case of when a magnitude of a constellation increases (e.g., in the case of 16-QAM or 64-QAM), since complexity of the system is increased, it is not possible to perform the ML detection in real time.
In order to provide optimal ML performance while the complexity of the system is reduced, a sphere decoding scheme has been proposed. In the sphere decoding scheme, since a test is performed on constellation points that exist near the received signals, complexity is alleviated as compared with the case of the simple ML detection method according to the related art.
However, when considering a downlink, the size of a mobile terminal is generally restricted. Therefore, it is difficult for a plurality of receiving antennas to be located at one mobile terminal. Actually, in the WiBro system, even though two or more receiving antennas are defined in one mobile terminal, a plurality of mobile terminals have a number of receiving antennas that is smaller than a number of transmitting terminals.
In particular, a scheme that is entitled “A new approach for fast generalized sphere decoding in MIMO systems” that was proposed by Z. Yang, C. Liu and J. He and disclosed in IEEE Signal Processing Letters, has been known as the fastest sphere decoding scheme when the number of receiving antennas is smaller than the number of transmitting antennas. However, as in the WiBro system, in the case where four transmitting antennas exist and the magnitude of the constellation is large, an algorithm is very complex. As a result, it is difficult to use the scheme in the system.
Examples of a method of reducing the complexity of a system and providing suboptimal performance include ZF (Zero-Forcing), MMSE (Minimum Mean Squared Error), SIC (ZF with Successive Interference Cancellation), MMSE with SIC, and the like. However, since these methods cannot provide the ML performance, they are inferior to the sphere decoding scheme that can ensure the ML performance.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.